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14.2
Chapter 12 (section 12.3.2) introduced the concept of broadcast networks
and the most common topologies. Remember that broadcast networks use a
channel to which all the users are connected, so all the users receive
any transmission made on the channel. The only wide area broadcast networks
all use radio broadcast, either relatively local up to a few hundred kilometres
or over much further distances using satellite transmission. Section 14.2.1
will examine this further. The most common examples of broadcast networks
are local area networks. These may use twisted pair cables, coaxial cable
or optical fibre cable as their transmission medium (see section 13.6).
Comparison between twisted pair and coaxial cable is not helpful because
there are many variants of each to meet the different requirements of
bandwidth, loss, noise immunity, etc. In general, coaxial cable has higher
noise immunity and bandwidth, but the cable is stiffer (which may or may
not be helpful depending on whether it is being surface mounted or pushed
through ducts). Both types can adequately serve most LAN environments,
but coaxial cable has been dropping out of use. Optical fibre is particularly
suited to environments which have high levels of electromagnetic radiation,
or to meet demands for very high speeds of transmission. However, it is
more difficult to tap into, which makes it more difficult and expensive
for the installation of a LAN.
LAN protocols have developed into the following layers:
physical layer
identical to ISO layer 1
medium access
control (MAC) layer to manage communications over the link
logical
link control (LLC) layer, which provides a form of multiplexing to
handle multiple-source data (a number of users attached to one host)
In addition, the
LLC layer assembles the data into a frame complete with address and
error checking bits and disassembles them on receipt.
For a particular LLC protocol there may be several different MAC options
provided, since this is the protocol layer in which the differences
in topology are involved.
The major standards activity for LAN networks has been by the US Institute
of Electrical and Electronic Engineers (IEEE). Their work has been
organised into a number of committees, of which some are as follows:
802.2 Logical
link control (LLC)
802.3 CSMA/CD
networks (Ethernet, etc.)
802.5
Token ring networks
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